Increase In Density Of States Research Articles

A regularized (XP)2 model

We investigate a dynamic model described by the classical Hamiltonian H(x, p) = (x 2 + a 2)(p 2 + a 2), where a 2 > 0, in classical, semi-classical, and quantum mechanics, respectively. In the high-energy E limit, the phase path resembles that of the XP2 model. However, the non-zero value of a acts as a regulator, removing the singularities that appear in the region where x, p ∼ 0, resulting in a discrete spectrum characterized by a logarithmic increase in state density. Classical solutions are described by elliptic functions, with the period being determined by elliptic integrals. In semi-classical approximation, we speculate that the asymptotic Riemann-Siegel formula may be interpreted as summing over contributions from multiply phase paths. We present three different forms of quantized Hamiltonians, and reformulate them into the standard Schrödinger equation with cosh2x -like potentials. Numerical evaluations of the spectra for these forms are carried out and reveal minor differences in energy levels. Among them, one interesting form possesses Hamiltonian in the Schrödinger equation that is identical to its classical version. In such scenarios, the eigenvalue equations can be expressed as the vanishing of the Mathieu functions’ value at i ∞ points, and furthermore, the Mathieu functions can be represented as the wave functions.

Experimental characterization of Spherical Bragg Resonators for electromagnetic emission engineering at microwave frequencies

This work reports experimental investigation and numerical validation of millimeter-sized Spherical Bragg Resonators (SBRs) fabricated using 3D printing technology. The frequency dependencies of the reflection and transmission coefficients were analyzed, and eigenfrequency values were calculated to examine the density of photonic states in air/PLA-polylactide SBRs, showing the appearance of an eigenmode and an increase in the local density of states in the core of a defect cavity. A decay rate enhancement of {sim 10}^{2} was obtained for a dipole placed in the core of the defect SBR. The study also investigated the influence of the source position on the resonator's electromagnetic wave energy. Scattering efficiencies up to order twelve of the multipole electric and magnetic contribution in a 10-layer SBR were calculated to validate the presence of the resonant modes observed in the scattering measurements performed for parallel and perpendicular polarizations. The results demonstrate that SBRs can act as omnidirectional cavities to enhance or inhibit spontaneous emission processes by modifying the density of electromagnetic states compared to free space. This finding highlights the potential of SBRs engineering spontaneous electromagnetic emission processes in various applications, including dielectric nanoantennas, optoelectronics devices, and quantum information across the entire electromagnetic spectrum.

Atomic-level insights into single-layer Co-promoted MoS2 clusters supported on a rutile TiO2(110) model oxide support

Single-layer mixed metal transition metal dichalcodenides (TMDCs), such as Co-promoted MoS2 supported on metal oxide supports like TiO2, have a wide range of applications in heterogeneous, electro- and photocatalysis, energy harvesting, and energy storage. In this work, we present atomic-level insights into the interaction between single-layer Co-promoted MoS2 clusters and a model oxide rutile TiO2(110) support. We have used physical vapor deposition (PVD) to synthesize a precursor containing bimetallic Co-Mo nanoparticles supported on (1 × 1) TiO2(110). We sulfide this precursor using an H2S background of 1 × 10−3 mbar at 650 K to synthesize the Co-promoted MoS2 clusters. We use a combination of scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to analyze the Co-promoted MoS2 clusters. We compare our results with that of pristine MoS2 clusters supported on TiO2(110), synthesized by an identical procedure, but without the Co atoms. We demonstrate that Co addition enhances the yield of (Co-promoted) MoS2 clusters, in comparison to sulfidation of Mo nanoparticles to form MoS2 clusters. Additionally, we also find that the Co-promoted MoS2 clusters have predominantly high-index edge terminations which likely are stabilized by Co-S-Ti and Mo-S-Ti linkages to an (3 × 1) S-TiO2 structure formed during the sulfidation. This causes an increase in the local density of states (LDOS) at the BRIM sites, thereby increasing their contrast in the STM images at the linkage sites. We have used our experimental results to propose a candidate atomic model for the Co-promoted MoS2 clusters supported on TiO2(110).

Crystal Structure, Electronic Structure, the Density of States, Optical Properties, and Superconducting Transition Temperature of ZrBeSi Crystal under Pressure

Through a rigorous application of first‐principles simulations, it is endeavored to provide a systematic examination of how the crystal structure, electronic structure, density of states, optical properties, and superconducting transition temperature of ZrBeSi are influenced by variations in pressure. The research has shown that pressure can alter the electronic structure and density of states, with a tendency for expansion toward higher energy regions as pressure increases. By manipulating the pressure, both the absorption coefficient and energy loss are sensitive to pressure and exhibit sharp absorption and loss peaks in the UV wavelength region. In addition to the above, the effects of electron–phonon coupling are taken into account and further investigations of the superconducting transition temperature , which is found to be 0.564 K at 0 GPa, are subsequently delved into. Additionally, there exists a quadratic attenuation relationship between the temperature and pressure. Further studies reveal that the decrease of with increasing pressure is a result of the combined effects of the gradual increase in the phonon density of states’ frequency and the flattening of the density of states near the Fermi level. The findings of this study contribute to the understanding of the impact of pressure on the physical properties of materials.

Bi-polaron hopping process of some MoO3 doped glassy system: Efors-Shkolvshkii gap in the unoccupied states

New glassy nanocomposites, yMoO3 - (0.1 – y)V2O5 – 0.9 (0.05ZnO - 0.95CuO) is found significant not only for surveying their microstructures using X-ray diffraction, FT-IR and SEM techniques, but also for setting out to explore their electrical conduction mechanism in terms of bi-polaronhopping process. Different nanophases such as Cu4O3, ZnO, Cu3Mo2O9 etc. have been identified and the average size of estimated nanocrystallites is found to increase with MoO3content.As MoO3 content is incorporated gradually, orthorhombic phases are found to be the major phases containing TMIs. Formation of more orthorhombic phases with higher MoO3 incorporation may directly indicate the stable structure of the resultant glassy nanocomposites.Electrical conductivity of the present system has been analyzed using bi-polaron hopping process, which may take place among various localized states in the extended stable orthorhombic structures. Significant increase in density of states near Fermi level, N(EF) at higher temperature may directly indicate the expansion of the present glassy structure with more and more MoO3incorporation. All experimental data have been used to frame a schematic model to explore conduction mechanism inside the present glassy system.

Effects of interstitial impurities (H, O, and He) on the structure and electronic properties of [formula omitted]–W: A density functional theory-based study

The A15 structure of W, the β–W, exhibits better properties for spintronics applications than bcc α–W. Previous works found that impurities could stabilize β–W. It is therefore important to identify the role and effects of impurities in the material. Hence, we performed DFT-based calculations and investigated the effects of interstitial impurities (12.50% and 1.56% atomic impurity concentrations of H, O, and He) on the structural and electronic properties of β–W. We also investigated α–W for comparison. We found that the binding energy of O is more favored in β–W than in α–W. However, the cohesive energy per atom of β–W is higher than α–W only when there is 12.50% atomic concentration of O in the system. This suggests that O has the tendency to induce α–W-to-β–W transition at this concentration of O. We found that the binding energies of H in β–W are energetically more favorable than in α–W, but the cohesive energies per atom are higher in α–W. These results imply that it will require a larger amount of energy to form β–W with H as an impurity. We found no stable site for He. Interaction between impurities and W atoms induces structural changes and an increase in the electron density of states at the Fermi level which highlights the effects of impurities in β–W.

Structural and optical properties of polyvinyl alcohol/copper oxide (PVA/CuO) nanocomposites

In the present investigation, pristine polyvinyl alcohol (PVA) and copper oxide (CuO) incorporated nanocomposite films have been fabricated via simple solution cast technique. The structural and optical properties of the prepared films were studied in detail. Various concentrations of CuO NPs (0.1–0.4 wt %) incorporated in the PVA film were prepared. The X-ray diffraction (XRD) profiles indicate the presence of CuO nanoparticles (NPs) in PVA film. XRD results indicate that, the CuO NPs were successfully incorporated into the PVA host matrix with no additional impurity peaks. The obtained CuO NPs belong to the monoclinic phase as confirmed by the XRD results. The peak observed at 2θ = 19.4° evident for the formation of semi crystalline PVA polymer. The average crystallite size of the prepared NPs was observed to be ∼0.52 nm. The vibrational bands present in the prepared films were confirmed by the Fourier transform infra-red spectroscopy (FTIR) plots. The presence of the NPs in the films was further investigated with the help of Raman spectra. UV–visible optical absorption spectra was recorded to evaluate the band gap values of the prepared films. The band gap value for pristine PVA was found to be 5.35 eV and it decreases with increase of CuO NPs concnetration owing to the change in the electronic structure of the pure PVA. The dielectric properties were also evaluated and it was found that, with increase in the CuO NPs concentration the dielectric constant also increases owing to the increase in density of states leading to the formation of polarization. The optical conductivity of the prepared nanocomposites increases with increase in the NPs concentration due to the formation of the charge transfer complex. The obtained results reveal that, the addition of CuO NPs reduces the optical band gap of the PVA polymer films.

Enhanced Magnetism in Heterostructures with Transition-Metal Dichalcogenide Monolayers.

Two-dimensional materials and their heterostructures have opened up new possibilities for magnetism at the nanoscale. In this study, we utilize first-principles simulations to investigate the structural, electronic, and magnetic properties of Fe/WSe2/Pt systems containing pristine, defective, or doped WSe2 monolayers. The proximity effects of the ferromagnetic Fe layer are studied by considering defective and vanadium-doped WSe2 monolayers. All heterostructures are found to be ferromagnetic, and the insertion of the transition-metal dichalcogenide results in a redistribution of spin orientation and an increased density of magnetic atoms due to the magnetized WSe2. There is an increase in the overall total density of states at the Fermi level due to WSe2; however, the transition-metal dichalcogenide may lose its distinct semiconducting properties due to the stronger than van der Waals coupling. Spin-resolved electronic structure properties are linked to larger spin Seebeck coefficients found in heterostructures with WSe2 monolayers.

Enhancing Critical Current Density of Bulk MgB2 via Nanoscale Boron and Dy2O3 Doping

Moderate critical current density (Jc) has been a long‐lasting problem in bulk MgB2 superconductors. We show a certain increment in Jc of bulk MgB2 via the use of amorphous boron precursor together with Dy2O3 doping. Dy2O3 dopant concentration varies from 0 to 2 wt%. X‐Ray diffraction (XRD) shows the formation of DyB4 particles. The critical temperature (Tc) is not affected by Dy2O3 doping and stands close to 38 K, showing that there is no Dy interaction with the MgB2 lattice. Microstructural studies show nanometer‐sized MgB2 grains. A high self‐field Jc of around 380 kA cm−2 is achieved at 20 K within the Dy2O3 doping range of 0.5–1.5 wt%. At around 1 wt% Dy2O3 doping an improved high‐field performance, 90 kA cm−2 at 2 T, 20 K, is observed. In the flux pinning diagram, 1 wt% Dy2O3 doping caused a peak shift from 0.19 (0 wt%) to 0.23. This indicates secondary pinning by DyB4 and lattice strains. Raman studies show the increase in the phonon density of states (PDOS) with increasing Dy2O3 doping.

Effects of interface charge-transfer doping on thermoelectric transport properties of black phosphorene-F4TCNQ nanoscale devices

Using first-principle-based density functional theory (DFT) combined with non-equilibrium Green's function (NEGF) method, we carry out a systematic study on thermoelectric transport properties of both armchair and zigzag black phosphorus nanoribbons (APNR and ZPNR) with physical adsorption of molecule F4TCNQ on their surfaces. The results indicate that APNR/F4TCNQ system is presented as a p-type semiconductor, and valence band edge of APNR system shifts to Fermi level (FL) due to surface charge transfer from APNR to F4TCNQ. Compared with pure APNR, thermal power S of APNR with F4TCNQ adsorption is increased from 270.8 μV/K to 1063 μV/K owing to the increase of state density induced from the emergence of resonance energy level near FL. Moreover, interface coupling and quantum interference effects inhibit phonon thermal conductance of A/ZPNR + F4TCNQ system significantly, and optimal thermoelectric figure of merit ZT of ZPNR/F4TCNQ system achieves 0.72 when μ = −0.5 eV at room temperature. This work provides a possibility for extensive application of high-performance two-dimensional organic–inorganic nanodevices in thermoelectric field, and it also has theoretical reference for developing self-assembled electronic device in practical applications.

Control of Water Adsorption via Electrically Doped Graphene: Effect of Fermi Level on Uptake and H2O Orientation

AbstractThe interaction of graphene with water molecules under an applied electric field is not thoroughly understood, yet this interaction is important to many thermal, fluidic, and electrical applications of graphene. In this work, the effect of electrical doping of graphene on water adsorption is studied through adsorption isotherms and current–voltage (IV) characterizations as a function of the Fermi level. The water adsorption onto graphene increases by ≈15% and the doping levels increase by a factor of three with a gate‐to‐graphene voltage of +20 or −20 V compared to 0 V for sub‐monolayer adsorption. This change in uptake is attributed to the increase in density of state of graphene upon electrical‐doping, which changes the Coulombic and van der Waals interactions. The water adsorption onto graphene is either n‐ or p‐doping depending on the applied gate‐to‐graphene voltage. The ambi‐doping nature of water onto graphene is due to the polar nature of water molecules, so the doping depends on the orientation of the water molecules.

Purcell effect of nitrogen-vacancy centers in nanodiamond coupled to propagating and localized surface plasmons revealed by photon-correlation cathodoluminescence

We measured the second-order correlation function of the cathodoluminescence intensity and investigated the Purcell effect by comparing the lifetimes of quantum emitters with and without metal structure. The increase in the electromagnetic local density of state due to the coupling of a quantum emitter with a plasmonic structure causes a shortening of the emitter lifetime, which is called the Purcell effect. Since the plasmon-enhanced electric field is confined well below the wavelength of light, the quantum emitter lifetime is changed in the nanoscale range. In this study, we combined cathodoluminescence in scanning (transmission) electron microscopy with Hanbury Brown-Twiss interferometry to measure the Purcell effect with nanometer and nanosecond resolutions. We used nitrogen-vacancy centers contained in nanodiamonds as quantum emitters and compared their lifetime in different environments: on a thin SiO2 membrane, on a thick flat silver film, and embedded in a silver film. The lifetime reductions of nitrogen-vacancy centers were clearly observed in the samples with silver. We evaluated the lifetime by analytical calculation and numerical simulations and revealed the Purcell effects of emitters coupled to propagating and localized surface plasmons. This is the first experimental result showing the Purcell effect due to the coupling between nitrogen-vacancy centers in nanodiamonds and surface plasmon polaritons with nanometer resolution.

Structural, electrical properties and stability in microwave dielectric properties of (1−x) MgTiO3-x SrTiO3 composite ceramics

Development of (1−x) MgTiO3 (MTO)-x SrTiO3 (STO) (for x = 0.04, 0.1–0.5) composite ceramics and tuning of temperature coefficient of resonant frequency (τf) near to zero through solid state reaction method have been reported. The MTO and STO crystals physically diffused together to form a single crystal structure without any trace of chemical interaction. A gradual improvement in dielectric constant (18.74–85.55) with an increase in STO content is obtained for the composite ceramics. Further, the dielectric constant remained almost invariant for a broad frequency range (100 Hz–100 MHz). However, temperature-dependent dielectric constant exhibited significant dispersion. The perusal of Mott’s variable range hopping 3-d model revealed the dominance of short-range hopping (<3 nm) in the composite ceramics. The activation energies for conduction are found to be decreasing (172–83 meV) with STO content which might be attributed to the increase in density of states from 1.65 × 1019 eV−1 cm−3 to 3.413 × 1020 eV−1 cm−3. The (1-x) MgTiO3-x SrTiO3 (for x = 0.04) composite ceramics exhibited best microwave dielectric properties with moderate quality factor (Q × f0) ~26,154 GHz at 8.08 GHz and better thermal stability (τf ~1.76 ppm/K). All the physical parameters followed cubic relation with STO content. The obtained microwave dielectric properties of 0.96MTO-0.04STO composite ceramics are suitable for microwave communication applications as microwave filters, resonators.

Atomistic insights into lithium adsorption and migration on phosphorus‐doped graphene

AbstractWe present a comprehensive density functional theory study of the effects of phosphorus doping of graphene on adsorption and diffusion behavior of lithium. We find that protrusion introduced by phosphorus doping enhances the adsorption of a single lithium atom onto graphene due to an additional partial covalent bonding between lithium and carbon atoms of the substrate. With an increase in concentration of lithium atoms, cluster formation is observed near the phosphorus dopant. Finite density of states at Fermi level indicates the electronic conductivity of phosphorus‐doped graphene before and after the adsorption of a lithium atom. An increase in density of states above the Fermi level is observed with increased lithium concentration. The protrusions caused by phosphorus dopants act as trapping centers for lithium, thereby hindering their migration over the substrate. This atomic level study can act as a guideline for further development of electrode materials for novel battery technologies.

Spontaneous emission dynamics of Eu3+ ions coupled to hyperbolic metamaterials

Sub-wavelength nanostructured systems with tunable electromagnetic properties, such as hyperbolic metamaterials (HMMs), provide a useful platform to tailor spontaneous emission processes. Here, we investigate a system comprising Eu3+(NO3)3·6H2O nanocrystals on an HMM structure featuring a hexagonal array of Ag nanowires in a porous Al2O3 matrix. The HMM-coupled Eu3+ ions exhibit up to a 2.4 fold increase in their decay rate, accompanied by an enhancement of the emission rate of the 5D0 → 7F2 transition. Using finite-difference time-domain modeling, we corroborate these observations with the increase in the photonic density of states seen by the Eu3+ ions in the proximity of the HMM. Our results indicate that HMMs can serve as a valuable tool to control the emission from weak transitions and, hence, hint at a route toward more practical applications of rare-earth ions in nanoscale optoelectronics and quantum devices.

Magnetic Ni doping induced high power factor of Cu2GeSe3-based bulk materials

Cu2GeSe3 is an eco-friendly material, but pristine Cu2GeSe3 has poor thermoelectric properties. Here, the effects of magnetic Ni ion with unpaired 3d electrons on the electrical and thermal transport properties of Cu2GeSe3 are reported. Density functional theory (DFT) calculations indicate that the unpaired Ni 3d states cause the hybridization of Ni 3d orbitals with Se 4p orbitals near the Fermi level, giving rise to an increase in density of states (DOS). Combined with the significantly increased carrier concentration, a power factor (S2σ) value of ∼8.0 μWcm−1 K−2 at 723 K is achieved in Cu2Ge0.8Ni0.2Se3 sample, seeming to be the highest compared with the other Cu2GeSe3 systems. Meanwhile, the substitution of Ni for Ge causes the obvious local distortions in Cu2GeSe3 lattice, which yields the large strain fluctuations to suppress the lattice thermal conductivity. Consequently, a peak ZT value of ∼0.38 at 723 K is obtained in Cu2Ge0.9Ni0.1Se3 sample.

Transition of charge transport phenomena from 3D to 1D hopping at low temperatures in polyaniline/graphene composites

Here, we present the charge transport properties of polyaniline/graphene composites prepared by a chemical oxidation method in the presence of four different loading concentrations (2, 4, 6, and 8 wt. %) of graphene. The synthesized materials are characterized for surface and chemical bonding analyses through field-emission scanning electron microscopy and Fourier transform infrared spectroscopy techniques, respectively. The change in the chemical structure of the prepared composites with graphene loading concentrations revealed the possible increment in electrical conductivity. The room-temperature dc conductivity of the prepared composites was found to increase from ∼22 to 217 S/cm with an increase in the loading concentration of graphene from 2 to 8 wt. %. The temperature-dependent electrical conduction behavior of the prepared samples is investigated under Mott's variable-range hopping conduction mechanism. It is found that all composite samples follow three-dimensional (3D) hopping in the higher temperature region (&amp;gt;44 K), which transforms into one-dimensional (1D) hopping at lower temperatures (&amp;lt;44 K). A decrease in hopping distance (1.07–0.96 nm) and an increase in density of states (3.20 × 1021–4.95 × 1021 cm−3 eV−1) in three dimensions with an increase in the graphene loading concentration from 2 to 8 wt. % suggest the requirement of lower hopping energy (61.3–55.5 meV) for conduction. The estimated hopping parameters also revealed a nonadiabatic small-polaron hopping conduction mechanism that is followed by the charge carriers in the present samples for both one- and three-dimensional variable range hoppings.

Discovery of AgxTaS2 superconductor with stage-3 structure

Through interfacial engineering, such as ion intercalation, we can tune the properties and optimize the performance of transition metal dichalcogenides and their devices. In this study, we present the first-time experimental investigations of stage 3 of AgxTaS2 single crystal, and its superconductivity was discovered at 3.8 K. Highly crystalline stage 1 and 2 were prepared by a proton-driven ion-introduction method. A simple water-soaking process was employed to achieve the stage-3 structure by deintercalation of Ag ions from lower stage structures. Besides, we developed a general stacking rule to determine the crystal structure, and it can predict any higher-order stage structure of AgxTaS2. The superconducting transition temperature was enhanced from 1.7 K for a stage-2 structure to 3.8 K for a stage-3 structure, which is more than four times that of the pristine TaS2 (0.8 K). This enhancement is attributed to the increase in density of states at the Fermi level, which was calculated by density functional theory. Also, the water-soaking process reconstructs the stage-2 into the stage-3 structure while deteriorating its crystallinity. Such a structural distortion is one of the potential reasons for the suppression of charge density wave in stage-2, resulting in the enhancement of superconductivity despite the structural degradation.

Electrochemically Desulfurized Molybdenum Disulfide (MoS₂) and Reduced Graphene Oxide Aerogel Composites as Efficient Electrocatalysts for Hydrogen Evolution.

Recent developments in graphene related materials including molybdenum disulfide (MoS₂) is gaining popularity as efficient and cost-effective nanoscale electrocatalyst essential for hydrogen production. These "clean" energy technologies require delicate control over geometric, morphological, chemical and electronic structure affecting physical and electrochemical catalytic properties. In this work, we prepared three-dimensional hierarchical mesoporous aerogels consisting of two-dimensional functionalized graphene and MoS₂ nanosheets of varying ratio of components under hydrothermal-solvothermal conditions (P <20 bar, T <200 °C). We systematically characterized these hybrid aerogels in terms of surface morphology, microstructure, understand heterointerfaces interaction through electron microscopy, X-ray diffraction, optical absorption and emission and Raman spectroscopy, besides electrochemical properties prior to and post electrochemical desulfurization that induces finely controlled sulfur vacancies. They feature enhanced electrical conductivity by means of eliminating contact resistance and meso-/nanoporous structure facilitating faster ion diffusion (mass transport). We demonstrate that controlled defects density, edges plane sites (nanowalls), mesoscale porosity and topological interconnectedness (monolithic aerogel sheets) invoked can accelerate electrocatalytic hydrogen production. For instance, low over potential with Tafel slope ~77 mV·dec-1 for 60 wt.% MoS₂, highcurrent density, and good stability was achieved with desulfurization. These results are compared with continuous multilayer MoS₂ films highlighting the multiple role of tunable structure and electronic properties. The adjacent S-vacancy defectsinduced increase in density of states, dissociation and confinement of water molecules at the pore edge and planar S-vacancy sites calculated using density functional theory helped in establishing improved heterogeneous electrocatalytic rate. This is supported with combined measurements of diffusion coefficient and heterogeneous electron transfer rate via surface-sensitive scanning electrochemical microscopy (SECM) technique.

Anion-site-modulated thermoelectric properties in Ge2Sb2Te5-based compounds

The amalgamation of multi-subjects often elicits novel materials, new concepts and unexpected applications. Recently, Ge2Sb2Te5, as the most established phase-change material, has been found to exhibit decent thermoelectric performance in its stable, hexagonal phase. The challenge for higher figure of merit (zT) values lies in reducing the hole carrier concentration and enhancing the Seebeck coefficient, which, however, can be hardly realized by conventional doping. Here in this work, we report that the electrical properties of Ge2Sb2Te5 can be readily optimized by anion-site modulation. Specifically, Se/S substitution for Te induces stronger and more ionic bonding, lowering the hole density. Furthermore, an increase in electronic density of state is introduced by Se substitution, contributing to a large increase in Seebeck coefficient. Combined with the reduced thermal conductivity, maximum zT values above 0.7 at 800 K have been achieved in Se/S-alloyed materials, which is ~ 30% higher than that in the pristine Ge2Sb2Te5.